1. Field of the Invention
This invention is directed to marine vibrators used in marine seismic systems. In particular, the invention is related to low frequency acoustic sources used in marine seismic surveys.
2. Description of Related Art
The prior art discloses a variety of marine vibrators (or underwater acoustic projectors) used in marine seismic surveys, including but not limited to U.S. Pat. Nos. 3,329,930, 3,384,868, 3,392,369, 3,394,775, 3,482,646, 3, 676,840, 4,853,905, and 4,885,726. Prior art vibrator devices have a variety of shapes ranging from cylindrical to bell-shaped to spherical. Operationally, these devices are sufficiently heavy so that they do not float and sink of their own accord.
The prior art discloses various underwater acoustic projectors (e.g. as in U.S. Pat. Nos. 3,875,552, 5,247,894 and 5,600,087) used in submarine warfare. These devices are streamlined, generally neutrally buoyant, have control systems which facilitate changing depth, course and speed, and may be remotely controlled or autonomous. They are small in size, operate in a frequency regime much higher than that useful to seismic exploration, and produce relatively low power acoustic signals.
In the marine seismic exploration industry, typical air gun projectors produce high powered acoustic signals (near 200 dB/Hz re .mu.Pa) in a low frequency band (5 Hz to 120 Hz). Existing prior art marine vibrators do not achieve these power levels in the low end of the seismic spectrum. Several existing prior art units may be used in concert to achieve the desired acoustic power in the seismic frequency band; but simultaneous use of multiple marine vibrator units complicates deployment, towing, synchronization, and operation.
If the acoustic projector is small compared to the wavelength of the signal being produced, the simple source equation (e.g. discussed in Theoretical Acoustics by Morse and Ingard, 1968) provides a model for predicting the performance of acoustic projectors. The simple source can be modeled as a sphere having the same surface area as the projector being modeled. It relates the acoustic pressure produced by a projector to the area, frequency and displacement of the projector surface. To produce a high-level acoustic pressure at low frequencies, a projector must have a large surface area which is displaced a relatively large distance. ##EQU1##
.rho.=density PA1 f=frequency PA1 A=projector area PA1 d=area displacement PA1 r=observation distance PA1 driving force, F=4 .pi..sup.2.rho.f.sup.2 Aad
Subsequently, the force required to move the large surface area a large distance is also large:
where a=radius of equivalent simple source sphere
In order for a single submersed marine vibrator to produce high-pressure, low-frequency acoustic signals, the surface area of the projector needs to be very large and the actuator which drives the project is relatively powerful.
Many prior art marine vibrators used in seismic survey operations minimize the enclosed volume of the device which results in strong negative buoyancy. The hydrodynamic drag coefficient of such prior art vibrators is relatively high.
There is a need for an effective and efficient marine vibrator useful in the seismic survey industry that is easily moved through water. Moreover, there is a need for a marine vibrator or an underwater acoustic projector operating in the seismic frequency band that has high power output (&gt;190 dB re .mu.pa) in the range of frequencies between 5 and 120 Hz, is lightweight to minimize difficulty in deployment and retrieval, is easily towed or pushed through the water, is capable of being steered to change depth and/or lateral position, and is remotely or autonomously controlled.